This invention relates to a two part piston with a rotatable piston top for an internal-combustion engine. The piston top comprises a piston head and the piston ring zone. The piston top is movable upward and downward with respect to the piston bottom connected with the connecting rod to obtain a variable compression height. The piston top is provided with a thread section which engages in a counterthread section on the piston bottom. Such a structure is generally known from the German Patent Document DE-PS 705 283.
The piston top in German Patent Document DE-PS 705 283, (which comprises the piston head) can be adjusted in the direction of the longitudinal axis of the piston with respect to the piston bottom connected with the connecting rod in order to change the compression ratio as a function of the operating condition of the internal-combustion engine. For this purpose, the piston top is connected with a rod which penetrates the piston bottom and has an external toothing. By the shifting of a toothed rack, which engages in the external toothing of the rod, the piston top can be rotated with respect to the piston bottom. The rotation of the piston top at the same time causes its vertical adjustment because the rod connected with the piston top has a thread which engages in a thread on the piston bottom. The described arrangement cannot be implemented in series production engines at reasonable cost because of the expensive and complicated mechanism.
In the case of a piston according to German Patent Application with the File Number P 40 18 344.0, which has not yet been published, the adjustment of the piston top with respect to the piston bottom takes place as a function of the inertia forces, the gas forces and the frictional forces. Pressure-medium-filled chambers and a pressure control valve are arranged in the piston. The compression ratio will change as a function of the resulting pressure medium pressure. In addition, the piston top is freely rotatably disposed with respect to the piston bottom. However, no possibility is indicated as to how the piston could be caused to perform a positive rotation which, as known, is advantageous with respect to the durability of the piston.
In the German Patent Document DE-AS 11 79 047, a piston is caused to perform a progressive rotating movement in order to achieve a lower wear and a lower tendency to scuff the piston. Because of the rotating movement, the piston bottom is kept free of deposits, such as carbon deposits. In order to permit the rotating movement, the piston is disposed by means of a spherical head. The compression height is unchangeable. The piston bottom is constructed in one piece with the piston skirt. The rotating movement of the piston is achieved by means of a mass which is caused, to perform a rotating movement by the axial movement of the piston. For this purpose, a threaded spindle is used on which a disk is arranged which is guided by means of a counterthread. Because of the axial acceleration forces of the piston, the disk experiences a helical rotating movement. Because of the effect of the inertia forces and frictional forces, a progressive rotating movement of the piston is caused when the normal braking movement of the disk differs in the upper and lower end position.
It is an object of the instant invention to cause a progressive rotating movement of the piston top as the piston top is adjusted in the longitudinal axis of the piston with respect to the piston bottom, the rotation of the piston top not being the cause for its stroke movement.
In order to achieve this object, in a piston with a variable compression height of the initially mentioned type, the invention provides two circumferences on the piston top with multiple threaded sections with opposed pitch and which engage in corresponding counterthread sections on the piston bottom. The position of the thread sections with respect to the longitudinal axis of the piston is a function of the maximally possible travel of the piston top part with respect to the piston bottom part. The matching thread sections and counterthread sections alternately engage, depending on whether the piston top moves upward or downward. Thus, during the reversal of the moving direction of the piston top, a moving-apart of the momentarily engaged thread sections and counterthread sections is possible. The thread sections and the counterthread sections rest only on one side of flanks situated in the moving direction. The maximally possible rotation of the piston top per stroke depends on the thread pitch. Corresponding to the thread pitch, an offsetting must also be provided either of the thread sections or of the counterthread sections so that the progressive rotation of the piston top is achieved.
It is further desireable if the flanks of the threaded sections and counterthreaded sections providing the engagement are constructed as lateral surfaces on radially projecting projections. Here the thread sections on the piston top part are located on a single projection of a series of projections and these projections are arranged between two rows of projections of the counterthread sections, whose flank are offset about the circumference of the piston bottom part. Alternatively the counterthreaded sections on the piston bottom part can be located on a single projection of a series of projections, and these projections are arranged between the two rows of projections of the threaded sections whose flank are offset about the circumference of the piston top part.
It is further desireable if a hydraulic device is provided for the adjustment of the piston top part with respect to the piston bottom part along the axis of the piston. The lateral surfaces of radial projections are used as thread flanks, in which case different lateral surfaces on one projection may represent thread flanks of a differing pitch. Particularly advantageously, the invention can be used in the pistons with a variable compression height in which hydraulic devices are used for the adjustment of the piston top.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.